Our research seeks to define the function of the dystrophin in normal tissues in order to explain how its absence or abnonnality leads to the pathologies observed in several muscular dystrophies and some cardiomyopathies. Dystrophin has been speculatively modeled as an antiparallel, rod-shaped dimer that crosslinks actin filaments by way of a putative actin-binding domain located at the amino-terminus of each dystrophin monomer. However, we have obtained biochemical evidence indicating that dystrophin exists as a monomer that binds alongside an actin filament via multiple contact points distributed throughout the amino-terminal and rod domains (Rybakova et al., 1996 J. Cell Biol., In Press). Our findings suggest that although dystrophin is most closely related to the actin crosslinking superfamily of proteins based on sequence homology, dystrophin functions more like an actin side binding protein such as tropomyosin. To further test the validity of this novel model for the interaction of dystrophin with F-actin, it is now necessary to obtain ultrastructural data on dystrophin when complexed with F-actin. Thus, we propose to visualize dystrophin/F-actin complexes by high resolution cryo scanning electron microscopy. We also propose to tag purified dystrophin with colloidal gold or with available site-specific antibodies in order to identify it in EM images. The results of this study are expected to provide the first convincing images of intact dystrophin and provide an independent test of our novel side-binding model for the interaction of dystrophin with F-actin.